Precision controlled precipitation of oxygen in silicon

ABSTRACT

Process for controlling the density of oxygen precipitate nucleation centers in single crystal silicon. In the process, the single crystal silicon is annealed at a temperature of at least about 350° C. to cause the formation of oxygen precipitate nucleation centers in the single crystal silicon. During the annealing step, the single crystal silicon is heated (or cooled) to achieve a first temperature, T 1 , which is between about 350° C. and about 500° C. The temperature is then increased from T 1  to a second temperature, T 2 , which is between about 500° C. and about 750° C. with the average rate of temperature increase from T 1  to T 2  being less than about 25° C. per minute. The annealing is terminated at a point in time when the oxygen precipitate nucleation centers are capable of being dissolved by heat-treating the silicon at a temperature not in excess of about 1150° C.

BACKGROUND OF THE INVENTION

The present invention relates generally to the fabrication of singlecrystal silicon substrates, and more particularly, to a process forfabricating single crystal silicon wafers having oxygen precipitatenucleation centers which can be stabilized and serve as a site for thegrowth of oxygen precipitates with the number of oxygen precipitateshaving a low order of dependance upon the oxygen concentration of thesingle crystal silicon.

Single crystal silicon, which is the starting material for mostprocesses for the fabrication of semiconductor electronic components, iscommonly prepared with the so-called Czochralski ("Cz") process whereina single seed crystal is immersed into molten silicon and then grown byslow extraction. At the temperature of the silicon molten mass, oxygencomes into the crystal lattice from the quartz crucible in which it isheld until it reaches a concentration determined by the solubility ofoxygen in silicon at the temperature of the molten mass and by theactual segregation coefficient of oxygen in solidified silicon. Suchconcentrations are greater than the solubility of oxygen in solidsilicon at the temperatures typical for the processes for thefabrication of integrated circuits. As the crystal grows from the moltenmass and cools, therefore, the solubility of oxygen in it decreasesrapidly, whereby in the resulting slices or wafers, oxygen is present insupersaturated concentrations.

Thermal treatment cycles which are typically employed in the fabricationof electronic devices can cause the precipitation of oxygen in siliconwafers which are supersaturated in oxygen. Depending upon their locationin the wafer, the precipitates can be harmful or beneficial. Oxygenprecipitates located in the active device region of the wafer can impairthe operation of the device. Oxygen precipitates located in the bulk ofthe wafer, however, are capable of trapping undesired metal impuritiesthat may come into contact with the wafer. The use of oxygenprecipitates located in the bulk of the wafer to trap metals is commonlyreferred to internal or intrinsic gettering ("IG").

Because of the problems associated with oxygen precipitates in theactive device region, electronic device fabricators must use siliconwafers which are incapable of forming oxygen precipitates anywhere inthe wafer under their process conditions, or alternatively, wafers whichonly form oxygen precipitates in the bulk of the wafer under theirprocess conditions. Many electronic device fabricators prefer the latteralternative in view of the benefits of IG.

In general, the electronic device fabrication process inherentlyincludes a series of steps which, in principle, can be used to form azone near the surface of the wafer which is free of oxygen precipitates(commonly referred to as a "denuded zone" or a "precipitate free zone")with the balance of the wafer containing a sufficient number of oxygenprecipitates for IG purposes. Denuded zones can be formed, for example,in a high-low-high thermal sequence such as (a) oxygen outdiffusion heattreatment at a high temperature (>1100° C.) in an inert ambient for aperiod of at least about 4 hours, (b) oxygen precipitate nucleiformation at a low temperature (600°-750° C.), and (c) growth of oxygen(SiO₂) precipitates at a high temperature (1000°-1150° C.). See, e.g.,F. Shimura, Semiconductor Silicon Crystal Technology, Academic Press,Inc., San Diego Calif. (1989) at pages 361-367 and the references citedtherein.

A critical requirement for many electronic device fabricators is thatall wafers subjected to their thermal sequence have a uniform andreproducible denuded zone and a uniform and reproducible number densityof oxygen precipitates outside the denuded zone. Uniformity andreproducibility have been difficult to achieve at a reasonable cost,however. There are several parameters which affect the density of oxideprecipitates which develop in a given silicon wafer in a given ICmanufacturing process, including: (1) the concentration of interstitialoxygen, [O_(i) ]_(i) present initially in solid solution, (2) thedensity of pre-existing (to the IC manufacturing process) oxygenclusters which act as nucleation sites for the precipitation ofsupersaturated oxygen, (3) the stability of these pre-existing clustersat higher temperatures, and (4) the details of the thermal cyclesemployed to produce the electronic device. These parameters can varysignificantly from one wafer to the next.

One approach which has been tried to control the range in theconcentration of oxygen precipitates formed during an IC manufacturingprocess is to narrow the range of oxygen concentration for the wafers.For example, many IC fabricators require that the range on oxygenconcentration be within 1 ppma of a target value, or even less. Thisapproach, however, stretches technological capability, reduces theflexibility of crystal growers to control other parameters and increasescosts. Even worse, tightening oxygen concentration specifications doesnot guarantee success; thermal histories of the silicon wafers can havea profound effect upon the oxygen precipitation behavior. Thus, wafershaving the same oxygen concentrations but different thermal historiescan exhibit significantly different precipitate densities.

In view of the fact that tightening oxygen concentration specificationsby itself will not lead to a narrow range of oxygen precipitatedensities, some have attempted sorting wafers by oxygen concentration orother criteria from which values of oxygen precipitation values can bepredicted. See, for example, Miller U.S. Pat. No. 4,809,196.Wafer-to-wafer uniformity with respect to oxygen precipitation isimproved by this approach, but flexibility is significantly impaired andcosts are increased.

Bischoff et al. suggest a process for forming wafers having a widedenuded zone (≧15 μm) with a high precipitate density (>10¹² /cm³) inU.S. Pat. No. 4,437,922. In their process, the denuded zone is formedfirst by annealing the wafers at 1100° C. for four hours. After thedenuded zone is formed, Bischoff et al. suggest that the wafers beannealed at temperature in the range of 400° to 500° C. to nucleate ahigh density of very small precipitates and grow them to such a size topermit survival of a subsequent heat treatment such as 925° C.Thereafter, Bischoff et al. suggest heating the wafers at a rate of lessthan 2° C. per minute to a temperature between 750° C. and 1000° C. andannealing the wafers at this temperature for a period which issufficient to ensure the survival of the precipitates in subsequentprocessing.

A significant disadvantage of the Bischoff et al. process is its failureto take advantage of the high temperature steps which are routinely usedin electronic device fabrication. These steps could be used to form thedenuded zone and to stabilize the oxygen precipitates in the balance ofthe wafer, provided the precipitation behavior were tailored for thespecific electronic device fabrication process. The additional stepsrequired by Bischoff et al. add significant labor and cost to thewafers. Furthermore, nowhere do Bischoff et al. disclose any means forcontrolling the number density of the oxygen precipitates; Bischoff etal. merely disclosed how to consistently obtain a high density (>10¹²/cm³) which may not be appropriate for many applications.

SUMMARY OF THE INVENTION

Among the objects of the invention, therefore, is the provision of aprocess for providing sets of single crystal silicon wafers which havebeen assembled without resort to sorting by oxygen concentration orthermal history and which, as a group, exhibit highly uniformprecipitation behavior; the provision of such a process in which theresulting wafers contain a substantially uniform distribution of oxygenclusters which, upon subsequent processing, can be dissolved to form adenuded zone in the region near the surface of the wafer and can bestabilized and serve as a site for the growth of oxygen precipitates inregions outside the denuded zone; and the provision of such a process inwhich the effective oxygen concentration dependence of the precipitationof oxygen in single crystal silicon is reduced.

Briefly, therefore, the present invention is directed to a process forcontrolling the density of oxygen precipitate nucleation centers insingle crystal silicon. The process comprises annealing the singlecrystal silicon at temperatures of at least about 350° C. The annealingcomprises (i) heat-treating the single crystal silicon at a temperature,T₁, between about 350° C. and about 500° C. to form oxygen precipitatenucleation centers in the single crystal silicon and (ii) increasing thetemperature of the single crystal silicon from T₁ to a secondtemperature, T₂, between about 500° C. and about 750° C. The rate oftemperature increase from T₁ to T₂ is controlled such that the singlecrystal silicon, upon achieving T₂, contains oxygen precipitatenucleation clusters formed at T₁. The annealing is terminated at a pointin time when the oxygen precipitate nucleation centers are capable ofbeing dissolved by heat-treating the silicon at temperatures not inexcess of about 1150° C.

The present invention is also directed to a process for controlling theconcentration of oxygen precipitate nucleation centers in single crystalsilicon in which the silicon is heat-treated to dissolve the oxygenprecipitate nucleation centers which were formed in the silicon duringthe growth of the single crystal. The heat-treated single crystalsilicon is then annealed at a temperature of at least about 350° C. tocause oxygen precipitate nucleation centers to be formed and thenincreased in size in the single crystal silicon with the annealingcomprising (i) causing the single crystal silicon to achieve a firsttemperature, T₁, which is between about 350° C. and about 500° C., and(ii) increasing the temperature of the single crystal silicon from T₁ toa second temperature, T₂, which is between about 500° C. and about 750°C. The average rate of temperature increase from T₁ to T₂ is controlledsuch that at an intermediate temperature, T_(int), between T₁ and T₂,the single crystal silicon contains a population of oxygen precipitatenucleation clusters at least some of which were present a temperaturewhich is less than T_(int) but greater than T₁ and which have increasedin size through the addition of oxygen atoms when the temperature wasincreased to T_(int).

The present invention is further directed to a process for controllingthe concentration of oxygen precipitate nucleation centers in a set ofat least 25 single crystal silicon wafers. Each of the wafers in the sethas a carbon concentration of less than 5×10¹⁶ atoms/cm³. The wafersalso have a concentration of oxygen falling within a range of oxygenconcentrations for the set with the difference between the maximum andthe minimum concentration in the range of oxygen concentrations for thewafers in the set being at least 5×10¹⁶ atoms/cm³. The process comprisesthe steps of (a) heating (or cooling) the single crystal silicon toachieve a first temperature, T₁, which is between about 350° C. andabout 500° C. The temperature is then increased from T₁ to a secondtemperature, T₂, which is between about 500° C. and about 750° C. at anaverage rate of less than 25° C. per minute. In addition, the increasein temperature from T₁ to T₂ is controlled such that the wafers in theset will contain oxygen precipitates upon being subjected to an oxygenprecipitation heat treatment with the ratio of the maximum density tothe minimum density of oxygen precipitates for the wafers in the setbeing no more than about 30.

The present invention is further directed to a set of at leasttwenty-five single crystal silicon wafers each of which contains asubstantially uniform oxygen concentration at distances greater than 3microns from the surface of the wafers. In addition, the differencebetween the maximum and the minimum concentration of oxygenconcentration for the wafers in the set is at least 5×10¹⁶ atoms/cm³.The wafers additionally contain oxygen precipitate nucleation centers ina density such that upon being subjected to an oxygen precipitation heattreatment, each of the wafers in the set will have a density of oxygenprecipitates with the ratio of the maximum density to the minimumdensity of oxygen precipitates for the wafers in the set being no morethan about 30.

Other objects and features of this invention will be in part apparentand in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the log of the oxygen precipitate density("OPD")/cm³ versus the initial oxygen concentration (atoms/cm³) for twosets of wafers subject to a series of heat treatments as set forth inExample 1.

FIG. 2 is a graph of the log of the oxygen precipitate density("OPD")/cm³ versus the the log of the initial oxygen concentration(atoms/cm³) for two sets of wafers, one subjected to an isothermalanneal and the other subjected to a ramp anneal as set forth in Example1.

FIG. 3 is a graph of the change in oxygen concentration ("DeltaOxygen")/cm³ versus the the log of the initial oxygen concentration(atoms/cm³) for a set of wafers subjected to a ramp anneal and a curveshowing what the relationship would have been if the wafers had beenisothermally annealed instead, as set forth in Example 2.

FIG. 4 is a graph which presents FIG. 3 and the data generated inExample 2 in a different form as described in Example 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Surprisingly, a process has been discovered for producing single crystalsilicon having any target oxygen precipitate density within a wide rangeof oxygen precipitate densities from silicon having any of a wide rangeof oxygen concentrations. In general, the target precipitate density mayfall anywhere within the range of about 10⁷ to about 10¹¹precipitates/cm³ and the oxygen concentration of the silicon in whichthis target is achieved may fall anywhere within the range of about6×10¹⁷ to about 8.5×10¹⁷ atoms/cm³ (ASTM standard F-121-83), i.e., therange of oxygen concentration which is typical of Cz grown silicon.

The process of the present invention is designed to utilize the hightemperature steps which are routinely used in electronic devicefabrication. Single crystal silicon which has been prepared by theprocess of the present invention contains oxygen precipitate nucleationcenters which have a low order of dependence upon oxygen concentration.When the silicon is subjected to such high temperature steps, the oxygenprecipitate nucleation centers will dissolve in a region near thesurface of the silicon to form a denuded zone whereas the oxygenprecipitate nucleation centers in the bulk of the silicon will bestabilized and grown into oxygen precipitates, the number density ofwhich is within a narrow range about a target density.

In accordance with the process of the present invention, a singlecrystal silicon starting material is subjected to an annealing treatmentwhich is referred to herein as ramped annealing. In this rampedannealing sequence, the single crystal silicon is heated to a firsttemperature, T₁, and the temperature of the silicon is then increased toa second temperature, T₂, with the rate of temperature increase betweenT₁ and T₂ being controlled to maintain and grow oxygen precipitatenucleation centers which have been formed in the single crystal siliconduring this process. The first temperature, T₁, is between about 350° C.and about 500° C., preferably between about 375° C. and about 475° C.,and most preferably, between about 400° C. and about 450° C. The secondtemperature, T₂, is between about 500° C. and about 750° C., preferablybetween about 575° C. and about 725° C., more preferably, between about600° C. and about 700° C., and most preferably, between about 600° C.and about 650° C.

The average rate at which the temperature of the silicon is increasedbetween T₁ and T₂ will vary depending upon the oxygen concentration ofthe silicon, but typically will be between about 0.2° and about 25° C.per minute. In general, the rate of temperature increase decreases withdecreasing oxygen concentration and thus, the average rate oftemperature increase may be less than 10° C. per minute, 5° C. perminute, or even less than 1° C. per minute for silicon having less thanabout 7.2×10¹⁷ oxygen atoms/cm³ (as determined in accordance with ASTMStandard F-121-83). The manner in which temperature is increased as afunction of time may be optimized for any given process; that is, it maybe increased linearly or in steps as a function of time and at varyingrates between the temperatures which are intermediate of T₁ and T₂.

The starting material for the process is single crystal silicon having asufficient concentration of oxygen to enable its precipitation whensubjected to an oxygen precipitation heat treatment. As previouslynoted, Czochralski grown silicon typically has an oxygen concentrationof about 6×10¹⁷ to about 8.5×10¹⁷ atoms/cm³ (ASTM standard F-121-83)which is sufficient for the precipitation of oxygen.

Oxygen can be precipitated from silicon, for example, by subjecting thesilicon to a heat treatment which comprises annealing the silicon at atemperature in the range of about 650° C. to about 875° C., preferablybetween about 800° C. and 875° C. to stabilize oxygen precipitatenucleation centers present in the silicon and annealing the silicon at atemperature of at least about 1000° C. to precipitate oxygen at the siteof the stabilized nucleation centers. The amount of time required tostabilize the nucleation centers is dependant upon the heat-treatingtemperature and, in general, less time is required to stabilize thenucleation centers as temperature is increased. For example, at leastabout six hours at a temperature of about 650° C. is required tostabilize the nucleation centers in silicon having an oxygenconcentration of about 7.8×10¹⁷ atoms/cm³, whereas only about one-halfhour is required to stabilize the nucleation centers at a temperature ofabout 800° C. A preferred heat treatment for precipitating oxygencomprises heating the wafers to 800° C. for four hours and then to 1000°C. for sixteen hours.

The single crystal silicon starting material should also have a lowdensity of, and preferably, an essential absence of oxygen precipitatenucleation centers. The presence (or density) of these nucleationcenters, however, cannot be directly measure using presently availabletechniques. As previously discussed, preexisting oxygen precipitatenucleation centers in the silicon can be stabilized and precipitates canbe grown at these sites by subjecting the silicon to an oxygenprecipitation heat treatment. Thus, the presence of these nucleationcenters can indirectly be measured after an oxygen precipitation heattreatment. The detection limit for oxygen precipitates is currentlyabout 10⁷ precipitates/cm³. As used herein, therefore, silicon having alow density of oxygen precipitate nucleation centers shall mean siliconwhich, upon being annealed at a temperature of 800° C. for four hoursand then at a temperature of 1000° C. for sixteen hours, has less thanabout 10⁸ oxygen precipitates/cm³. Similarly, silicon having anessential absence of oxygen precipitate nucleation centers shall meansilicon which, upon being annealed at a temperature of 800° C. for fourhours and then at a temperature of 1000° C. for sixteen hours, has lessthan 10⁷ oxygen precipitates/cm³. Preferably, the silicon has less than5×10⁷ oxygen precipitates/cm³, and more preferably the silicon has lessthan 10⁷ oxygen precipitates/cm³.

Oxygen precipitate nucleation centers typically form in silicon which isannealed at a temperature in the range of about 350° to about 750° C.The single crystal silicon starting material, therefore, may be silicontaken from "short" crystals, that is, silicon which has been grown in aCzochralski process until the seed end has cooled from the melting pointof silicon (1410° C.) to about 750° C. after which the ingot is rapidlycooled. In this way, the time spent in the temperature range criticalfor nucleation center formation is kept to a minimum and the nucleationcenters have inadequate time to form in the crystal puller.

Alternatively, and more preferably, the single crystal starting materialis annealed to dissolve the oxygen precipitate nucleation centers whichwere formed during the growth of the single crystal. Provided they havenot been subjected to a stabilizing heat-treatment, oxygen precipitatenucleation centers can be annealed out of silicon by rapidly heating thesilicon to a temperature of at least about 875° C. and preferablycontinuing to increase the temperature to at least 1000° C. By the timethe silicon reaches 1000° C., substantially all (e.g., >99%) of suchdefects have annealed out. It is important that the wafers be rapidlyheated to these temperatures, i.e., that the rate of temperatureincrease be at least about 10° C. per minute and more preferably atleast about 50° C. per minute. Otherwise, some or all of the oxygenprecipitate nucleation centers may be stabilized by the heat-treatment.Equilibrium appears to be reached in relatively short periods of time,i.e., on the order of 1 minute. Accordingly, oxygen precipitatenucleation centers in the single crystal silicon starting material maybe dissolved by annealing the silicon at a temperature of at least about875° C. for a period of at least about 30 seconds, preferably at leastabout 10 minutes. The dissolution may be carried out in a conventionalfurnace or in a rapid thermal annealing (RTA) system. In addition, thedissolution may carried out on crystal ingots or on wafers.

Substitutional carbon, when present as an impurity in single crystalsilicon has the ability to catalyze the formation of oxygen precipitatenucleation centers. For this and other reasons, therefore, it ispreferred that the single crystal silicon starting material have a lowconcentration of carbon. That is, the single crystal silicon should havea concentration of carbon which is less than about 5×10¹⁶ atoms/cm³,preferably which is less than 1×10¹⁶ atoms/cm³, and more preferably lessthan 5×10¹⁵ atoms/cm³.

The single crystal silicon should also preferably have a substantiallyuniform oxygen concentration as a function of depth from the siliconsurface. Unlike prior methods used to control the precipitation ofoxygen in silicon, it is unnecessary to subject the single crystalsilicon to a high temperature step to cause the outdiffusion of oxygenin regions near the surface of the silicon prior to the ramped annealingsteps of the present invention. Such high temperature steps when carriedout for the sole purpose of forming a denuded zone add significant costto the silicon wafer. Thus, it is preferred that the single crystalsilicon starting material for the ramped annealing sequence of stepshave a uniform concentration of oxygen from the center of the silicon toregions of the wafer which are within 15 microns of the silicon surface,more preferably from the center of the silicon to regions of the waferwhich are within 10 microns of the silicon surface, even more preferablyfrom the center of the silicon to regions of the wafer which are within5 microns of the silicon surface and most preferably from the center ofthe silicon to regions of the wafer which are within 3 microns of thesilicon surface. In this context, substantially uniform oxygenconcentration shall mean a variance in the oxygen concentration of nomore than about 50%, preferably no more than about 20% and mostpreferably no more than about 10%.

When single crystal silicon starting material having a substantiallyuniform oxygen concentration is subjected to the ramped annealingsequence of the present invention, single crystal silicon having acorresponding uniform density of oxygen precipitate nucleation centerswill be produced. That is, after ramped annealing the single crystalsilicon will have a substantially uniform density of oxygen precipitatenucleation centers from the center of the silicon to regions of thewafer which are within 15 microns of the silicon surface, morepreferably from the center of the silicon to regions of the wafer whichare within 10 microns of the silicon surface, even more preferably fromthe center of the silicon to regions of the wafer which are within 5microns of the silicon surface and most preferably from the center ofthe silicon to regions of the wafer which are within 3 microns of thesilicon surface. In this context, substantially uniform density ofoxygen precipitate nucleation centers shall mean a variance in thenumber density of no more than a factor of 10, preferably no more than afactor of 5 and most preferably no more than a factor of 2.

The ramped annealing process of the present invention is distinguishablefrom conventional isothermal annealing processes. In a conventionalisothermal annealing process, the single crystal silicon is heated to atarget temperature within the range of about 500° C. to about 650° C.,annealed at that target temperature for a period of about fifteenminutes to several hours or more, and then cooled. During the annealingperiod, oxygen precipitate nucleation centers are formed. Based uponexperience to date, a plot of the precipitate density versus initialoxygen concentration for a set of wafers which have been isothermallyannealed will generally show an approximate dependence of [O_(i) ]^(n)where n is between about 25 and 30, provided the wafers have anessential absence of oxygen precipitate nucleation centers prior to theisothermal anneal.

If ramped annealing is used instead of isothermal annealing, however,the oxygen precipitate dependencies upon oxygen concentration of singlecrystal silicon can be significantly reduced. That is, the exponent "n"can be reduced to less than 25, preferably less than 20, more preferablyless than 15 and optimally less than 10.

Without being bound to any theory, it is believed that the differencebetween ramped annealing and isothermal annealing can be explained onthe basis of the effect of concentration and temperature upon theprecipitation of oxygen. In general, the more solute atoms that areinvolved in a particular sized cluster, the stronger is the dependenceon the concentration of the solute on the probability of its forming. Tothe first order, the nucleation rate at any one temperature for such acluster goes as [O_(i) ]^(x) where x is the number of atoms in thecluster. Also, it takes more time to build larger cluster than smallerones and the time it takes to build large clusters increasesdramatically with decreasing temperature. Combining these observationsabout the oxygen dependence of small clusters and the time dependence oflarge cluster formation at low temperatures suggests that at lowtemperatures the population of oxygen clusters is completely dominatedby clusters of the lowest order--clusters which have formed with arelatively weak dependence on the concentration of oxygen present in thecrystal. Some or all of these type of clusters may be observable as"thermal donors."

The ramped annealing process of the present invention, begins with arelatively low first temperature, T₁, designed to create only very smallclusters, that is, clusters which form with a relatively weak oxygenconcentration dependence. The larger clusters which are necessary forthe nucleation of oxide precipitation form as the temperature is slowlyramped up. Larger clusters grow by building preferentially onpre-existing smaller clusters which were formed with a relatively weakdependence on the oxygen concentration. If enough time is allowed for asufficient quantity of very low order clusters to form and if the rampup in temperature is done sufficiently slowly to avoid too muchdissolution of these defects as temperature increases, the weakdependence on oxygen concentration at the first temperature, T₁, can bepassed on to some extent to the larger defects. Stated another way, therate of temperature increase from T₁ to T₂ is controlled such that at anintermediate temperature, T_(int), between T₁ and T₂, the single crystalsilicon contains a population of oxygen precipitate nucleation clustersat least some of which, preferably the majority of which, were presentat a temperature which is less than T_(int) but greater than T₁ andwhich increased in size through the addition of oxygen atoms when thetemperature was increased to T_(int) (the balance of the population ofoxygen precipitate nucleation centers which are present at T_(int) beingspontaneously formed at each T_(int)).

The result of ramped annealing is a population of larger clusters whichexist at each intermediate temperature, T_(int), and ultimately at thesecond temperature, T₂, whose density dependence on oxygen concentrationis greatly reduced as compared to isothermal annealing. Significantly,these clusters are not of such a great size that their survival insubsequent device processing is insured; they can be dissolved bysubsequent thermal processing. As a result, wafers subjected to rampedannealing and thereafter to the high-low-high thermal process stepswhich typically occur in the fabrication of an electronic device willnaturally and desirably form a denuded zone near the surface of thewafer and a target density of oxygen precipitates outside the denudedzone. In addition, the range of precipitation densities for a givenoxygen content is readily adjustable with a wide variety of parameterswhich allow for the setting of both an average target value and theslope of the oxygen concentration dependence. These parameters are:

1) the first temperature, T₁ ;

2) the length of time for which T₁ is maintained;

3) the ramp rate to higher temperatures;

4) the second temperature, T₂ ; and

5) the length of time for which T₂ is maintained.

In general, the first temperature, T₁, is maintained for a period of onehour or less whereas the second temperature, T₂, is maintained for aperiod which is determined by the magnitude of the second temperature.If T₂ is between about 600° C. and 650° C., T₂ will typically bemaintained for no more than four hours, preferably for no more thanabout two hours and more preferably for no more than about one hour. IfT₂ is between about 650° C. and 700° C., T₂ will typically be maintainedfor no more than two hours, preferably for no more than about one hourand more preferably for no more than about one-half hour. If T₂ isbetween about 700° C. and 750° C., T₂ will typically be maintained forno more than one hour, preferably for no more than about one-half hourand more preferably for no more than about one-quarter hour.

As previously noted, the average ramp rate between T₁ and T₂ will bebetween about 0.2° C. and 25° C. per minute. Importantly, the rampbetween any two temperatures in the temperature range of T₁ to T₂ shouldbe controlled to maintain and grow oxygen precipitate nucleation centerswhich existed at a lesser temperature.

After the single crystal silicon is subjected to ramped annealing, it isimportant that the silicon not be subjected to a heat-treatment whichstabilizes oxygen precipitation nucleation centers prior to aheat-treatment which causes the formation of a denuded zone. Otherwise,the oxygen clusters (oxygen precipitate nucleation centers) which werenucleated and grown during the ramped annealing step would be stabilizedsubstantially throughout the entire thickness of the silicon andeffectively prevent the formation of a denuded zone during a subsequentelectronic device fabrication process.

It is worth emphasizing that oxygen precipitate nucleation centersformed in single crystal silicon by ramped annealing in accordance withthe present invention are capable of being dissolved. As a result, thenucleation centers in the region near the surface of the wafer can bedissolved to form a denuded zone during a thermal processing step whichis a natural and inherent part of an electronic device fabricationprocess. Outside this denuded zone, the oxygen precipitate centers canbe stabilized and serve as the site for the growth of oxygenprecipitates in later processing steps with the number density of theprecipitates outside the denuded zone being within a relatively narrowrange about a target value.

In addition, the number density of the precipitates outside the denudedzone for a set of wafers can be controlled to have a low order ofdependence upon oxygen concentration. Advantageously, therefore, sets ofwafers sliced from one or more crystal ingots grown by the Czochralskitechnique can be prepared which have a relatively narrow range ofdistribution about a target oxygen precipitate number density withoutresort to sorting based upon oxygen concentration or thermal history, orby requiring that the silicon contain unacceptably high carbonconcentrations. For example, a set of wafers (e.g., at least 2 andpreferably at least about 25-200 wafers) sliced from single crystalsilicon grown by the Czochralski technique and subjected to rampedannealing and then to an oxygen precipitate stabilization and growthheat treatment can have an oxygen precipitate density within a narrowrange about a target density which may be anywhere between about 10⁷ toabout 10¹¹ precipitates/cm³. The distribution of the wafers in the setabout this target density will be a function of the order of dependenceupon oxygen concentration and the range of oxygen concentrations of thewafers in the set and can be mathematically expressed as ##EQU1##wherein O_(i) ^(T) =initial oxygen concentration target value for awafer population,

O_(i) ^(T) +δO_(i) =upper limit to oxygen concentration in waferpopulation,

O_(i) ^(T) -δO_(i) =lower limit to oxygen concentration in waferpopulation,

OPD^(T) =target precipitate density for the value of O_(i) ^(T),

OPD^(T) +δOPD⁺ =maximum oxygen precipitate density in population,

OPD^(T) -δOPD⁻ =minimum oxygen precipitate density in population, and##EQU2## In general, the ratio of maximum to minimum precipitate valuesin a population of wafers for a given O_(i) ^(T) ±δO_(i) wherein O_(i)^(T) is any value which can be achieved with Czochralski grown siliconand δO_(i) is between about 0.5 ppma and 2 ppma will desirably be morethan 100, preferably is less than 50, more preferably is less than 30,and for some applications, most preferably is less than 20, 15, 10 oreven 5. Presented in Tables I-III are calculated values for thepredicted maximum to minimum precipitate values in a population ofwafers (hereinafter the "OPD max/min ratio") for selected orders ofdependence upon oxygen concentration, target oxygen concentration, andranges of oxygen concentration about the specified target, for thewafers in the population. The target oxygen concentration and range ofoxygen concentrations is expressed in ppma with 1 ppma being equivalentto 5×10¹⁶ atoms/cm³.

                                      TABLE I                                     __________________________________________________________________________      O.sub.i.sup.T = 12 ppma                                                               O.sub.i.sup.T = 13 ppma                                                               O.sub.i.sup.T = 14 ppma                                                               O.sub.i.sup.T = 15 ppma                                                               O.sub.i.sup.T = 16 ppma                                                               O.sub.i.sup.T = 17 ppma               δO.sub.i = 1 ppma                                                               δO.sub.i = 1 ppma                                                               δO.sub.i = 1 ppma                                                               δO.sub.i = 1 ppma                                                               δO.sub.i = 1 ppma                                                               δO.sub.i = 1 ppma               OPD max/min                                                                           OPD max/min                                                                           OPD max/min                                                                           OPD max/min                                                                           OPD max/min                                                                           OPD max/min                         m ratios  ratios  ratios  ratios  ratios  ratios                              __________________________________________________________________________    10                                                                              5.32    4.67    4.18    3.80    3.50    3.25                                15                                                                              12.25   10.10   8.55    7.41    6.54    5.85                                20                                                                              28.25   21.82   17.50   14.45   12.22   10.55                               25                                                                              65.13   47.17   35.78   28.17   22.85   19.00                               30                                                                              150.15  101.95  73.19   54.92   42.73   34.24                               __________________________________________________________________________

                                      TABLE II                                    __________________________________________________________________________      O.sub.i.sup.T = 12 ppma                                                               O.sub.i.sup.T = 13 ppma                                                               O.sub.i.sup.T = 14 ppma                                                               O.sub.i.sup.T = 15 ppma                                                               O.sub.i.sup.T = 16 ppma                                                               O.sub.i.sup.T = 17 ppma               δO.sub.i = 1.5 ppma                                                             δO.sub.i = 1.5 ppma                                                             δO.sub.i = 1.5 ppma                                                             δO.sub.i = 1.5 ppma                                                             δO.sub.i = 1.5 ppma                                                             δO.sub.i = 1.5 ppma             OPD max/min                                                                           OPD max/min                                                                           OPD max/min                                                                           OPD max/min                                                                           OPD max/min                                                                           OPD max/min                         m ratios  ratios  ratios  ratios  ratios  ratios                              __________________________________________________________________________    10                                                                              12.34   10.16   8.59    7.44    6.56    5.87                                15                                                                              43.37   32.36   25.20   20.29   16.79   14.21                               20                                                                              152.37  103.13  73.86   55.34   42.99   34.42                               25                                                                              535.32  328.67  210.54  150.93  110.09  83.37                               30                                                                              1880.76 1047.38 634.82  411.63  281.90  201.93                              __________________________________________________________________________

                                      TABLE III                                   __________________________________________________________________________      O.sub.i.sup.T = 12 ppma                                                               O.sub.i.sup.T = 13 ppma                                                               O.sub.i.sup.T = 14 ppma                                                               O.sub.i.sup.T = 15 ppma                                                               O.sub.i.sup.T = 16 ppma                                                               O.sub.i.sup.T = 17 ppma               δO.sub.i = 2 ppma                                                               δO.sub.i = 2 ppma                                                               δO.sub.i = 2 ppma                                                               δO.sub.i = 2 ppma                                                               δO.sub.i = 2 ppma                                                               δO.sub.i = 2 ppma               OPD max/min                                                                           OPD max/min                                                                           OPD max/min                                                                           OPD max/min                                                                           OPD max/min                                                                           OPD max/min                         m ratios  ratios  ratios  ratios  ratios  ratios                              __________________________________________________________________________    10                                                                              28.93   22.23   17.76   14.62   12.34   10.63                               15                                                                              155.57  104.83  74.83   55.92   43.37   34.67                               20                                                                              836.68  494.28  315.34  213.85  152.37  113.04                              25                                                                              4499.88 2330.58 1328.83 817.79  535.32  368.60                              30                                                                              24201.43                                                                              10988.98                                                                              5599.67 3127.28 1880.76 1201.91                             __________________________________________________________________________

Based upon the data presented in Tables I-III, therefore, sets of wafershaving an oxygen concentration of 12±1 ppma, 13±1 ppma, 14±1 ppma, 15±1ppma, 16±1 ppma, or 17±1 ppma may be prepared with the ratio of themaximum to minimum oxygen precipitate density for the wafers in the setbeing less than 50, 40, 30, 20, 15, 10 or even 5. Similarly, sets ofwafers having an oxygen concentration of 12±1.5 ppma, 13±1.5 ppma,14±1.5 ppma, 15±1.5 ppma, 16±1.5 ppma, or 17±1.5 ppma, may be preparedwith the ratio of the maximum to minimum oxygen precipitate density forthe wafers in the set being less than 100, 50, 40, 30, 20, 15, 10 oreven 5. In addition, sets of wafers having an oxygen concentration of12±2 ppma, 13±2 ppma, 14±2 ppma, 15±2 ppma, 16±2 ppma, or 17±2 ppma maybe prepared with the ratio of the maximum to minimum oxygen precipitatedensity for the wafers in the set being less than 200, 150, 100, 50, 40,30, 25, 15, 10 or even 5. These sets of wafers may be contained, forexample, in wafer cassettes of the type in which wafers are typicallystored and shipped, in boats of the type which are typically used forheat-treating silicon wafers, or in an equivalent wafer carrier.

The following examples will illustrate the invention.

EXAMPLE 1

To demonstrate the difference between the ramped annealing process ofthe present invention and isothermal annealing, two groups of siliconwafers having a wide range of oxygen concentrations taken from the samecrystals were treated. The wafers in both groups were heated to 1000° C.for 15 minutes to dissolve the oxygen precipitate nucleation clustersformed during the growth of the crystals. One of the groups was thenisothermally annealed at 500° C. for 2 hours, and the other group wasramped annealed by heating the wafers to 450° C. for 30 minutes,increasing the temperature of the wafers at the rate 1° C. per minuteuntil the wafers reached 650° C., and maintaining them at 650° C. for 15minutes. Thereafter, the wafers in both groups were heated to 800° C.for 4 hours and then to 1000° C. for 16 hours to stabilize and growoxygen precipitates (the "N1 treatment"). The results are presented inFIGS. 1 and 2.

The improvement of the ramped annealing treatment on the isothermaltreatment is remarkable. The oxygen precipitate density ("OPD") of theisothermally annealed group shows an approximate 32th power dependenceon initial oxygen concentration ("O_(i) ") whereas the ramp annealedmaterial shows only a 13th power dependence. An effect of this reductionis, for example, that a variation in precipitation within a ±1 ppmaoxygen concentration is reduced from about two orders of magnitude toabout a factor of 2, i.e., an improvement by a factor of 50.

EXAMPLE 2

The procedures of Example 1 were repeated except that three sets ofwafers were sliced from three different crystals (45DSE, XE8999, andXEH001) and two sets of wafers were sliced from different portions of afourth crystal (22FWJ, and 22FWA) and none of the wafers were subjectedto an isothermal anneal. The change in oxygen concentration ("deltaoxygen") as a result of this anneal as a function of the initial oxygenconcentration is presented in FIG. 3. Also presented in FIG. 3 is agenerally smooth, S-shaped curve which depicts what the change in oxygenconcentration would be as a function of the initial oxygen concentrationif the wafers had been isothermally annealed, the curve being based uponconsiderable prior experimental data. The improvement of rampedannealing over isothermal annealing is evident from this data.

EXAMPLE 3

Another way of illustrating the advantages of the invention using thedata generated in Example 2 appears in FIG. 4. A hypothetical oxygenprecipitation requirement of delta [O_(i) ]=1.5×10¹⁷ to 4.5×10¹⁷ cm³following the N1 treatment (see Example 1) was established. As thefigure illustrates, such a specification could only be met in standardisothermal annealed material by using a first oxygen specification of±0.3 ppma, which is practically infeasible with conventional crystalpulling technology. Using the ramped annealing approach, however, thesame tight oxygen precipitation specification could be met with a firstoxygen specification of ±1 ppma which is routinely achieved usingconventional crystal pulling technology. For less stringentspecifications on oxygen precipitation, the first oxygen concentrationspecification could even be widened beyond ±1 ppma and still achievesignificantly better performance than the standard production methodscurrently used today.

Further refinements in the starting temperature, ramp rate, and secondtemperature relative to the conditions employed in Example 2 couldresult in further improvements such that, at least theoretically, theexponent of the oxygen concentration dependence could be reduce to suchan extent that given the relatively narrow range of oxygen concentrationwhich the Czochralski process is capable of giving, that the need tomeasure or even specify first oxygen concentration could becomeirrelevant. All material inside this range (all Cz material) would beeffectively identical (and identically adjustable) in performance. Assuch, crystal growth processes could be designed and optimized, entirelyignoring the question of oxygen.

In view of the above, it will be seen that the several objects of theinvention are achieved.

As various changes could be made in the above compositions and processeswithout departing from the scope of the invention, it is intended thatall matter contained in the above description be interpreted asillustrative and not in a limiting sense.

What is claimed is:
 1. A process for controlling the density of oxygenprecipitate nucleation centers in single crystal silicon comprising thesteps of:annealing the single crystal silicon at temperatures of atleast about 350° C., the annealing comprising (i) heat-treating thesingle crystal silicon at a temperature, T₁, between about 350° C. andabout 500° C. to form oxygen precipitate nucleation centers in thesingle crystal silicon and (ii) increasing the temperature of the singlecrystal silicon from T₁ to a second temperature, T₂, between about 500°C. and about 750° C., and (iii) controlling the rate of temperatureincrease from T₁ to T₂ such that the single crystal silicon, uponachieving T₂, contains oxygen precipitate nucleation clusters formed atT₁, and terminating said annealing at a point in time when the oxygenprecipitate nucleation centers are capable of being dissolved byheat-treating the silicon at a temperature not in excess of about 1150°C.
 2. The process of claim 1 wherein the single crystal silicon has acarbon concentration of less than 1×10¹⁶ atoms/cm³.
 3. The process ofclaim 2 wherein the single crystal silicon is not heat-treated to form adenuded zone prior to said annealing step.
 4. The process of claim 1wherein the single crystal silicon is not heat-treated to form a denudedzone prior to said annealing step.
 5. The process of claim 1 wherein T₁is between about 375° C. and about 475° C. and T₂ is between about 575°C. and about 725° C.
 6. The process of claim 1 wherein average rate oftemperature increase from T₁ to T₂ is less than 25° C. per minute. 7.The process of claim 1 wherein average rate of temperature increase fromT₁ to T₂ is less than 10° C. per minute.
 8. The process of claim 1wherein average rate of temperature increase from T₁ to T₂ is less than5° C. per minute.
 9. The process of claim 1 wherein average rate oftemperature increase from T₁ to T₂ is less than 1° C. per minute.
 10. Aprocess for controlling the concentration of oxygen precipitatenucleation centers in single crystal silicon comprising the steps of:(a)heat-treating single crystal silicon which contains oxygen precipitatenucleation centers formed during the growth of the single crystal at atemperature of at least about 875° to dissolve the oxygen precipitatenucleation centers without forming a denuded zone, and (b) annealing theheat-treated single crystal silicon at a temperature of at least about350° C. to cause oxygen precipitate nucleation centers to be formed andthen increased in size in the single crystal silicon, the annealingcomprising (i) causing the single crystal to achieve a firsttemperature, T₁, which is between about 350° C. and about 500° C., and(ii) increasing the temperature of the single crystal silicon from T₁ toa second temperature, T₂, which is between about 500° C. and about 750°C. with the average rate of temperature increase from T₁ to T₂ beingcontrolled such that at each intermediate temperature, T_(int), betweenT₁ and T₂, the single crystal silicon contains a population of oxygenprecipitate nucleation clusters at least some of which were present atemperature which is less than T_(int) but greater than T₁ and whichhave increased in size through the addition of oxygen atoms when thetemperature was increased to T_(int).
 11. The process of claim 10wherein the single crystal silicon has a carbon concentration of lessthan 1×10¹⁶ atoms/cm³.
 12. The process of claim 10 wherein T₁ is betweenabout 375° C. and about 475° C. and T₂ is between about 575° C. andabout 725° C.
 13. The process of claim 10 wherein average rate oftemperature increase from T₁ to T₂ is less than 10° C. per minute.
 14. Aprocess for controlling the concentration of oxygen precipitatenucleation centers in a set of at least 25 single crystal silicon waferseach of which has a carbon concentration of less than 5×10¹⁶ atoms/cm³and a concentration of oxygen falling within a range of oxygenconcentrations defined by the set with the difference between themaximum and the minimum concentration in the range of oxygenconcentrations for the wafers in the set being at least 5×10¹⁶atoms/cm³, the process comprisingcausing the wafers to achieve a firsttemperature, T₁, which is between about 350° C. and about 500° C. andincreasing the temperature of the wafers from T₁ to a secondtemperature, T₂, which is between about 500° C. and about 750° C., withthe average rate of temperature increase from T₁ to T₂ being controlledsuch that (i) at each intermediate temperature, T_(int), between T₁ andT₂, the single crystal silicon contains a population of oxygenprecipitate nucleation clusters at least some of which were present atemperature which is less than T_(int) but greater than T₁ and whichhave increased in size through the addition of oxygen atoms, and (ii)after the temperature of the wafers is increased to T₂, the wafers havea density of oxygen precipitate nucleation centers such that upon beingsubjected to an oxygen precipitation heat treatment consistingessentially of annealing the wafers at 800° C. for four hours and thenat 1000° C. for sixteen hours, the wafers in the set contain oxygenprecipitates with the ratio of the maximum density to the minimumdensity of oxygen precipitates for the wafers in the set being no morethan about
 50. 15. The process of claim 14 wherein the density of oxygenprecipitates for the wafers in the set upon being subjected to theoxygen precipitation heat treatment is between about 10⁷ and about 10¹¹precipitates/cm³.
 16. The process of claim 14 wherein the differencebetween the maximum and the minimum concentration in the range of oxygenconcentrations for the wafers in the set is at least 7.5×10¹⁶ atoms/cm³.17. The process of claim 14 wherein the difference between the maximumand the minimum concentration in the range of oxygen concentrations forthe wafers in the set is at least 1×10¹⁷ atoms/cm³.
 18. The process ofclaim 14 wherein said process is terminated at a point in time when theoxygen precipitate nucleation centers formed during the process arecapable of being dissolved by heat-treating the wafers at a temperaturenot in excess of about 1150° C.
 19. The process of claim 14 wherein saidwafers are not heat-treated to form a denuded before being caused toachieve the first temperature, T₁.
 20. The process of claim 14 whereinaverage rate of temperature increase from T₁ to T₂ is less than 10° C.per minute.
 21. The process of claim 14 wherein the ratio of the maximumdensity to the minimum density of oxygen precipitates for the wafers inthe set is no more than about
 30. 22. The process of claim 14 whereinthe ratio of the maximum density to the minimum density of oxygenprecipitates for the wafers in the set is no more than about
 20. 23. Theprocess of claim 14 wherein the ratio of the maximum density to theminimum density of oxygen precipitates for the wafers in the set is nomore than about
 10. 24. The process of claim 14 wherein the ratio of themaximum density to the minimum density of oxygen precipitates for thewafers in the set is no more than about
 5. 25. A set of single crystalsilicon wafers assembled in a wafer cassette, boat or other wafercarrier which have not been selected for inclusion in the set on thebasis of oxygen concentration or thermal history, each of the singlecrystal silicon wafers in the set having a substantially uniform oxygenconcentration at distances greater than 5 microns from the surface ofthe wafers with the difference between the maximum and the minimumconcentration of oxygen concentration for the wafers in the set being atleast 5×10¹⁶ atoms/cm³, each of the wafers in the set additionallyhaving a concentration of oxygen precipitate nucleation centers suchthat upon being subjected to an oxygen precipitation heat treatmentconsisting essentially of annealing the wafers at 800° C. for four hoursand then at 1000° C. for sixteen hours, the wafers in the set containoxygen precipitates with the ratio of the maximum density to the minimumdensity of oxygen precipitates for the wafers in the set being no morethan about
 50. 26. The set of wafers of claim 25 wherein the set ofwafers comprises at least about 25 wafers.
 27. The set of wafers ofclaim 25 wherein the difference between the maximum and the minimumconcentration in the range of oxygen concentrations for the wafers inthe set is at least 7.5×10¹⁶ atoms/cm³.
 28. The set of wafers of claim27 wherein the ratio of the maximum density to the minimum density ofoxygen precipitates for the wafers in the set is no more than about 20.29. The set of wafers of claim 25 wherein the ratio of the maximumdensity to the minimum density of oxygen precipitates for the wafers inthe set is no more than about
 20. 30. The set of wafers of claim 25wherein the ratio of the maximum density to the minimum density ofoxygen precipitates for the wafers in the set is no more than about 10.31. The set of wafers of claim 25 wherein the ratio of the maximumdensity to the minimum density of oxygen precipitates for the wafers inthe set is no more than about
 5. 32. The set of wafers of claim 25wherein the difference between the maximum and the minimum concentrationin the range of oxygen concentrations for the wafers in the set is atleast 1×10¹⁷ atoms/cm³.
 33. The set of wafers of claim 32 wherein theratio of the maximum density to the minimum density of oxygenprecipitates for the wafers in the set is no more than about 20.